Conventionally, a so-called multi-throttle valve is known in which throttle valves are disposed in individual intake passages for each cylinder of an engine. The multi-throttle valve linearly adjusts the amount of intake air according to driver's demand for example, thereby improving acceleration response. Moreover, a rotary-type throttle valve is known in which its valve body has a rotatable and cylindrical shape and is formed with an intake passage therein, while being located closely to a combustion chamber in an intake passage. (refer to Japanese publication of patent application no. 2001-73813) This throttle valve should be located as closely as possible to the engine, on the downstream side of the intake passage, for further improvement in acceleration response, because this arrangement reduces the volume of the portion of the intake passage downstream of the throttle valve, thereby improving acceleration response. Additionally, a so-called inertia effect of the intake flow is known which improves volumetric efficiency of intake air to generate high engine output. The effect is attained by: an intake passage comprising a low-speed passage and a high-speed passage for supplying intake air from a surge tank to a combustion chamber; and a switching valve provided at a collecting portion of the passages.
The switching valve changes the length of the intake passage in accordance with the engine rotational speed so that the frequency of the intake pressure fluctuation caused by the descending motion of a piston during the intake stroke (the value associated with the engine rotational speed) is synchronized with the natural frequency of the intake system determined from the length of the intake passage and the volume of the cylinder, thereby attaining the inertia effect of the intake flow (refer to Japanese granted patent application no. 5-78651, for example.)
In the case where the engine, utilizing the inertia effect of the intake flow, is equipped with the throttle valve close to its combustion chamber, at least two valves, (i.e., the switching valve for controlling the inertia effect of the intake flow and the throttle valve) are located closely to the combustion chamber or the engine body. This causes problems such as a considerably complex structure with difficulty in the layout of the intake system.